Color is one of the most important quality factors of edible oils. Consumers generally prefer edible oils with a light color, since light color has been traditionally associated with a fresh, high quality plant oil. However, crude plant oils may have dark color associated with impurities and color bodies. To produce a light-colored plant oil, impurities and color bodies may be removed from the crude oil during refining or production.
Many of the color bodies present in plant oils occur naturally. As used herein, the terms “color body” and “color bodies” refer to a group(s) or molecule(s) present in an edible oil composition which imparts a color to the edible oil composition. Non-limiting examples of naturally occurring color bodies include carotenes, xanthophylls, and chlorophylls, including derivatives of chlorophylls, such as, pheophytin. In addition to naturally occurring color bodies, other color bodies may be generated during processing or refining of the plant oil. For example, certain color bodies in crude plant oils, such as, for example, crude corn oil, may be formed as a result of the Maillard reaction of various oil components during high temperature thermal treatment of the oilseeds prior to oil extraction. Other color bodies may develop due to mishandling of the plant oils or the oilseeds. Also, additional color bodies may be generated as oilseeds age before processing. Color bodies generated during processing, by mishandling, or by aging may be difficult to remove by conventional decolorization methods.
In addition to the color bodies described above, under certain conditions, plant oils that have been processed to yield a light-colored product may become darker upon storage. This darkening of plant oils during storage is known as “color reversion” in the industry. The degree of color change during storage may be dependent on various factors, such as, for example, raw seed quality, refining conditions, moisture content of oilseeds or refined oils, and storage conditions. For example, oxidation or polymerization of tocopherols, such as gamma-tocopherol, into colored compounds is believed to be a factor in the color reversion in some oils. Color reversion of oils may also vary according to oil type, with some oils requiring only a few hours to revert and other oils requiring up to several months. For example, refined corn oil is known in the industry to have a notoriously short color reversion induction time. Producing color-stable plant oils has been a major interest of the edible oil industry.
Oil colors are difficult to define and to measure. Several color measurement methods are currently used within the industry. One method of measuring the color of an oil is by color matching. For example, color matching using the Lovibond method (see, American Oil Chemist's Society (“AOCS”) Method Cc 13b-45, Color Wesson Method Using Color Glasses Calibrated in Accordance with the AOCS-Tintometer Color Scale, the disclosure of which is incorporated in its entirety by reference herein) involving the use of yellow-tinted and red-tinted Lovibond glasses has been practiced for many years in the plant oil industry. More recently, automated Lovibond colorimeters have been developed to measure red and yellow color values based on an oil sample's absorption of light at specific wavelengths. Other methods of color measurement are also used in the industry.
Crude oils are typically refined prior to use. As used herein, the term “refining” refers to any purifying treatment designed to remove at least one of free fatty acids, phosphatides, color bodies, and other impurities from the oil. The refining of edible oils typically involves several steps. First the oils are extracted from the plant source, generally via pressing, for example using a continuous screw press, or by extraction with volatile solvents. For animal fats, rendering is used to separate the fat from the fatty tissues to obtain lard or tallow. The resulting oils and fats are comprised of triacylglycerols which are trimesters of fatty acids with glycerol. As used herein, the terms “triacylglycerols”, “triacylglycerides”, “triglycerides”, “acyl glycerides”, and “glycerides” are used interchangeably and refer to the tri-esters of fatty acids with glycerol. Refining of the edible oils may be via physical or chemical refining, or a combination of the two approaches.
Physical refining consists of removing the fatty acids from the crude oil by steam distillation under a vacuum after the phosphatides have been removed by a degumming process. Chemical refining, the conventional method for removal of the nonglyceride impurities from edible fats and oils, consists of a combination of optional degumming, alkali-refining, bleaching, and deodorizing. In chemical refining, alkali-refining (also known as caustic neutralization) is used to remove the free fatty acids from the crude oil in the form of soapstock. Alkali-refining may also remove phosphatides in the form of coagulated masses which may also entrain insoluble matter, and certain oil pigments may be degraded, absorbed or made water soluble by the alkali. The resulting soapstock and coagulated masses may be removed, for example, by centrifugation. Degumming typically involves mixing the crude oil composition with water to remove water soluble components. Degumming and alkali-refining may be done simultaneously in a process known as “crude-refining”. Alkali-refining of glyceride oils may also be done in miscella in a process called miscella refining. As used herein, the term “miscella” means a mixture of the glyceride oil and a volatile organic solvent, such as, for example, hexane. In miscella refining, the oil/solvent miscella is refined by chemical refining, for example, by degumming (optionally), alkali-refining, bleaching, and deodorization. Miscella refining of oil has advantages over conventional refining techniques, such as, lower refining loss, better separation of the soapstock from the miscella mixture, and lighter-colored refined oil.
As discussed herein, conventional chemical and physical oil refining processes typically involve bleaching and deodorizing steps. Bleaching of edible fats and oils is regarded as the removal of non-glyceride impurities, including color bodies, from the composition. Although oils may be bleached chemically, the color reduction occurs generally by oxidation reactions which may have undesirable effects on the flavor and/or oxidative stability of the oil and, consequently, chemical bleaching is not used for edible oils. Conventional bleaching is by adsorption of the color bodies and other non-glyceride impurities, such as, metals, soaps, and phosphatides, on bleaching earth. Examples of bleaching earths include natural earths and clays, activated earths and clays, and activated carbon. Generally, the bleaching materials are added to the oil in a vessel followed by agitation, either at atmospheric pressure or under reduced pressure. The oil may be heated to a bleaching temperature and held, to allow contact time with the bleaching earth. After sufficient time has passed, the bleaching earth is removed from the oil, for example, by filtration or centrifugation.
Acid-activated bleaching clay has been used in conventional bleaching processes. For example, one commonly used acid-activated clay bleaching method is described in U.S. Pat. No. 4,443,379. While clay may be an efficient adsorbent for some natural color pigments, such as chlorophylls, bleaching clay is also known to catalyze some undesirable reactions during the bleaching process that may create some color bodies, known as “fixed color”. Fixed color is not readily removed during the oil refining process and may impart an undesirable color to the oil. In addition, the use of higher dosages of bleaching clay may cause a quicker color reversion upon storage of the oil. Other adsorbents, such as acid-activated silica (U.S. Pat. No. 4,877,765), have been used for the removal of color bodies as well as other trace contaminants. However, silica has typically not provided very light color for corn oil. Activated carbon may remove some color bodies that are not easily removed by bleaching clays. However, activated carbons are expensive and are not effective in removing all types of color bodies, for example, certain color bodies present in corn oil.
Natural fats and oils may retain certain undesirable odors and flavors after refining. Thus, deodorizing may be necessary. Deodorizing is used to provide the bland flavor and odor expected by consumers and generally involves a high-temperature, high-vacuum, steam distillation process. During deodorization, further removal of certain color bodies may occur by heat bleaching of the color bodies.
Even though the bleaching and deodorization process removes color bodies, the process may also negatively affect the color of the bleached oil, especially when bleaching with acid-activated clay. Certain oil components, such as tocopherols, may be oxidized during the bleaching step, thereby generating precursors of color reversion. These precursors may be further oxidized during bleaching to become reverted color pigments. The reverted color pigments may be converted back to the precursors during deodorization due to the high heat resulting in an oil with an initially acceptable color. However, during storage of the deodorized oil, the precursors may again be converted back to color bodies resulting in color reversion. The extent of color reversion may be related to the dosage of bleaching clay, which is generally related to the degree of secondary oxidation of the oil, as measured by the para-anisidine value of the bleached oil. Other disadvantages of clay bleaching include the disposal of the spent clay, increased oil loss in the spent clay, and the fire hazard associated with spontaneous ignition of the spent bleaching clay.
Corn oil is obtained from corn germs, which are separated from corn kernels during wet corn milling. Dried corn germs contain about 50% oil by weight. In commercial settings, the germs are first pressed (expelled) to squeeze the oil out from the germ meal and then the meal may be further extracted with a non-polar solvent for maximum recovery of the crude corn oil. The ratio of expelled oil to extracted oil may be approximately 3:2.
Refined corn oil is known for its darker color. The dark color of refined corn oil is due, at least in part, to Maillard reaction products that are generated during corn processing before oil extraction. For example, melanoidins, such as, nitrogenous polymers, are dark brown colored substances generated by the Maillard reaction of amino acids and carbohydrates in the corn. Melanoidins are thought to be some of the unbleachable color pigments in finished corn oil. Corn oil color bodies are generally not easily removed from the oil during conventional refining processes and, as a result, conventionally refined corn oil will have a golden color. Corn oil is also known in the oil industry for its fast color reversion. The unbleachable color bodies and fast color reversion has prevented the vegetable industry from making light-colored corn oil with acceptable color stability. A stable and light-colored corn oil could be used in a wide range of food application where the darker color of corn oil has been an obstacle.
Because some of the unbleachable color bodies in the corn oil are believed to arise from abuse of the corn germ during wet milling, there have been efforts to process corn in a way that minimizes the damage during the corn processing prior to oil extraction. For example, U.S. Pat. No. 6,388,110 (“the '110 Patent”) discloses a dry corn milling process that yields a lighter colored crude corn oil. The crude corn oil produced by this method was reported to have a lighter color than crude corn oil produced from conventional wet-milling processes. However, the '110 Patent does not disclose refining the crude corn oil to actually produce a finished light-colored, color stable, edible corn oil. U.S. Pat. No. 4,808,426 discloses a corn oil extraction process that yields a light-colored crude corn oil. However, the process utilizes vegetable oil as the extraction medium, which results in a high vegetable oil content in the corn meal.
Ion exchange compounds, such as resins, are generally classified according to three criteria: the nature of their functional groups; the chemistry of the matrix supporting the functional group; and the porosity of the matrix supporting the functional group (“Ion Exchange” in Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition (vol. 14), p b737-741; John Wiley & Sons, New York, 1995). The four primary types of ion exchange functionality are strong acid, weak acid, weak base, and strong base. Strong acids or bases are generally differentiated from weak acids or bases by the ability of strong acids and bases to split neutral salts, such as sodium chloride. Solid ion exchangers are most often formed as resins. Acidic ion exchange resins, also known as cationic ion exchange resins, are often in the hydrogen form for use. In this form, when contacted with a liquid containing cations, hydrogen cations associated with the ion exchange resin are able to leave the solid phase and enter the liquid phase as they are exchanged with cations in the liquid phase. Basic ion exchange resins, also known as anionic ion exchange resins, are often in the hydroxide form. In this form, when contacted with a liquid containing anions, hydroxide anions associated with the ion exchange resin are able to leave the solid phase and enter the liquid phase as they are exchanged with anions in the liquid phase. In addition, other ion exchange resins may have the ability to chelate metals. Ion exchange interactions are reversible, which may allow regeneration procedures to return an ion exchanger to the desired form for reuse.
Anionic ion exchange resins are generally amine-based resins. Strong base anion exchange resins have functional groups comprising quaternary ammonium hydroxide groups. Weak base anion exchangers typically have functional groups comprising primary, secondary, or tertiary amines.
The use of basic, anionic ion exchange resins in a plant oil refining method is disclosed in U.S. Pat. No. 2,771,480 (“the '480 Patent”). The '480 Patent discloses refining plant oils with a basic anion exchange resin to reduce free fatty acids and some color pigments in the oil. The anionic exchange resin used in the '480 Patent is a strong base anion exchange resin containing quaternary ammonium hydroxide groups. The anionic exchange resin was primarily employed to remove free fatty acids. The anionic resin refining method of the '480 Patent was presented as an alternative to alkali-refining of the oil. Decolorization of the oil was an additional benefit of the process. Detailed measurement of the reduction in color was not presented.
Thus, conventional methods of refining and bleaching plant oils, and in particular, corn oil, do not provide a light-colored refined oil with color stability. New methods for producing light-colored oils with color stability are desired.